A multi-centre observational study investigating the prevalence of spurious hyperkalaemia due to potassium ethylenediaminetetraacetic acid (kEDTA) contamination. Serum EDTA was measured in anonymised serum samples with a serum potassium > 6.0 mmol/L collected over a one month period in five different hospital laboratories. Two of the participating laboratories routinely screen all hyperkalaemic samples for EDTA contamination. EDTA contamination was present in 4.1% (range 1.2%-6.7%) of hyperkalaemic samples. In three laboratories, without routine EDTA screening, 50% "EDTA contaminated" were identified by laboratory staff, the remaining 50% samples were undetected and reported as genuine hyperkalaemia. In these laboratories, EDTA was not measurable in 2 samples reported as "EDTA contaminated". Spurious hyperkalaemia due to kEDTA contamination is relatively common. Education regarding correct blood collection technique offers the best strategy in preventing EDTA sample contamination. Gross kEDTA contamination is easily identified by laboratory staff in samples with marked unexpected hyperkalaemia and hypocalcaemia. Spurious hyperkalaemia due to modest kEDTA contamination may only be confidently detected by measurement of serum EDTA.